I have 3 T-SQL queries(SELECT statements) all operating on same set of two tables on SQL Server 2008 R2 but with different search criterias(WHERE clauses).I wish to make a single query by suitably combining these queries so that it returns a table containing 4 columns.

Move all the conditions that are not common to all three into CASE expressions - like shown below.

SELECT a.NAME,
SUM(
CASE
WHEN TRANSTYPE = 1 AND REF1 = 'invoice' AND
NOT (t.INVNO LIKE 'CR%' OR REF2 LIKE 'T#%') THEN t.AMOUNT
ELSE 0
END) AS LifeTimeInvoice,
ABS(SUM(
CASE
WHEN TRANSTYPE IN (4, 5) --Payment
AND REF1 <> 'Credit' THEN t.AMOUNT
ELSE 0
END )) AS LifeTimePayment,
SUM(
CASE
WHEN (TRANSTYPE IN (4, 5) AND REF1 = 'Credit')
OR (TRANSTYPE = 1 AND (t.INVNO LIKE 'CR%' OR REF2 LIKE 'T#%'))
THEN t.AMOUNT
ELSE 0
END ) AS LifeTimeCredit
FROM dbo.DR_ACCS a
JOIN dbo.DR_TRANS t
ON a.ACCNO = t.ACCNO
WHERE a.CustomerCode = 1326499
GROUP BY
a.NAME

SELECT
a.NAME,
SUM(CASE WHEN TRANSTYPE = 1 AND REF1 = 'invoice' AND NOT (t.INVNO LIKE 'CR%' OR REF2 LIKE 'T#%') THEN t.AMOUNT ELSE 0 END) AS LifeTimeInvoice,
SUM(CASE WHEN TRANSTYPE IN (4, 5) AND REF1 <> 'Credit' THEN t.AMOUNT ELSE 0 END) AS LifeTimePayment,
SUM( CASE WHEN ((TRANSTYPE IN (4, 5) AND REF1 = 'Credit') OR (TRANSTYPE = 1 AND (t.INVNO LIKE 'CR%' OR REF2 LIKE 'T#%'))) THEN t.AMOUNT ELSE 0 END) AS LifeTimeCredit
FROM dbo.DR_ACCS a
JOIN dbo.DR_TRANS t
ON a.ACCNO = t.ACCNO
WHERE a.CustomerCode = 1326499
GROUP BY a.NAME